1. Field of the Invention
This invention is related to a method of converting digital data when recording and playing back digitalized TV video signals and especially to a method of recording and playing back data of medical image diagnostic apparatus, etc.
2. Description of the Invention
Recently, as computers have become popular, medical image diagnostic apparatuses with a computer have been employed so that diagnostic image data has become digitalized. Initially, CT (computed tomography), for still pictures were made and recently DSA (digital subtraction angiography) etc. for moving pictures, have appeared (U.S. Pat. No. 4,204,225, U.S. Pat. No. 4,204,226).
FIG. 1 shows an image input system of a DSA. The X-rays generated in the X-ray source are transmitted through the subject 2 and enter the image intensifier 3 to produce an X-ray image. The image intensifier 3 projects a circular visible image corresponding to the X-ray image. The TV camera 4 picks up the entire circular visible image, converts it to a video signal and outputs. The video signal obtained in this manner is treated in various ways and recorded on an analog VTR, digital disc recorder, etc.
The visible image projected on the image intensifier 3 used for this DSA is circular, while the image pickup (scanning) region is approximately rectangular (as in usual TV pictures). Therefore, the circular visible image region should be contained within the image pickup region (regular square). In this case, assuming the radius of the visible image as A, the area where the image pickup region and visible image region are overlapped, that is, the available image pickup (scanning) region (.pi.A.sup.2) containing image information is about 80% of the entire image pickup region (4A.sup.2). The residual area of about 20% is unavailable image pickup region containing no image information for this part of the video signal. Since the image signal converted with the TV camera 4 contains this unavailable video signal, the DSA treats the unnecessary signal also and records it.
In usual TV systems, image signals are digitalized in 8 or less number of bits when recorded. In the case of images for medical diagnosis, higher resolution than in usual TV pictures is required and the signals should be quantized at more than 9 or 10 bits. Therefore, data processing at higher speed and larger capacity than in usual TV signals is required. Such unavailable video signals containing no information as image signals are usually quantized at more than 9 or 10 bits and then the quantized signals are processed and recorded.
As described above, treating and recording unavailable video signals containing no image information and available video signals containing video signals together lead to functional deterioration of the entire system because of the unavailable video signals. Especially when these video signals are quantized and transmitted or recorded and played back, there are difficulties with respect to recording capacity, etc.